1. Field of the Invention
The invention is related to the field of petroleum wellbore perforating using high explosive shaped charges. More specifically, the invention is related to safety devices to prevent unintended detonation of an explosive oil well perforating gun at any place other than the selected depth in the wellbore.
2. Description of the Related Art
Petroleum wellbore perforating uses high explosive shaped charges to create "perforations" in steel casing included in completed wellbores. The shaped charges are detonated by a signal from a detonating cord attached to or in close proximity to the shaped charges. The detonating cord is itself initiated by a blasting cap, exploding bridgewire or similar initiator. See for example, U.S. Pat. No. 5,533,454 issued to Ellis et al for a description of an exploding bridgewire firing circuit, and U.S. Pat. No. 4,428,440 issued to McPhee for a description of a perforating gun assembly including shaped charges, charge carrier, detonating cord and initiator.
Various safety systems have been developed to reduce the chance of unintended detonation of the shaped charges by accidental firing of the initiator. For example, the exploding bridgewire firing system described in the Ellis et al '454 patent is intended to reduce the possibility of unintended detonation of the shaped charges as a result of stray radio frequency energy firing a blasting cap initiator. Other systems for preventing unintended detonation include key-locking switches which shunt the electrical cable connected to the perforating "gun" assembly (as described for example in the Ellis et al '454 patent) through resistors to prevent electric charge build up on the cable from accidentally initiating the blasting cap or exploding bridgewire circuit.
More recently, users of oil well perforating equipment have developed a technique known as "electric before ballistic arming" to reduce the consequences of unintended initiation of the blasting cap or exploding bridgewire ("initiator"). Generally speaking, in this technique all electrical connections between the initiator and the electrical cable (used to lower the gun into the wellbore) are made prior to attaching the initiator to the detonating cord. This procedure is based on the probability that any accidental firing of the initiator will take place immediately or shortly after the electrical connections are made. If the initiator is not yet attached to the detonating cord at the time of accidental firing, the consequences of the unintended firing will be limited to the damage caused solely by the initiator's release of energy, which is relatively small. Otherwise, the full energy of the exploding shaped charges would be accidentally released at the earth's surface, with a high probability of loss of life and severe damage to property.
The "electric before ballistic arming" safety technique is impracticable to use on perforating gun assemblies which exceed the lifting height of equipment located at the well site, however. This equipment includes drilling rigs or completion/workover rigs as is known in the art. These types of rigs typically have a maximum lifting height of about 90 feet. It has become quite common to perforate wellbores using a single perforating gun assembly having much more widely spaced apart perforation intervals, and consequently much greater overall gun length, than the 90 foot lifting height of the typical drilling or workover rig. See for example U.S. Pat. No. 5,533,454 issued to Ellis et al for a description of a perforating gun assembly having multiple perforation intervals.